1. Field of the Invention
The present invention is related to automobile exhaust particulate filters, and in particular to automobile particulate filters that are used in diesel-powered automobiles.
2. Background Art
Emissions of nitrogen oxide (“NOx”) and particulate matter (“PM”) are of primary concern for both diesel and gasoline vehicles to meet future emissions standards. Diesel vehicles have significant advantages over their gasoline counterparts including a more efficient engine, higher fuel economy, and lower emissions of HC, CO, and CO2. For example, diesel vehicles potentially have a 40% higher fuel economy than current gasoline vehicles with 20% lower CO2 emissions.
Aftertreatment systems for diesel engines typically use separate devices for reduction of NOx and particulate matter emissions to meet regulatory standards. For example, a filter composed of a ceramic monolith traps PM and the PM is oxidized on a periodic basis. The PM is trapped by plugging every other channel of the filter substrate with nonporous material thereby forcing the exhaust gas through the filter wall. Typical ceramics used to make the filter substrate are cordierite and silicon carbide.
Available technologies for NOx reduction in lean environments include Selective Catalytic Reduction (“SCR”), in which NOx is continuously removed through active injection of a reductant over a catalyst and Lean NOx Traps (LNT), which are materials that adsorb NOx under lean conditions and must be periodically regenerated by running under rich conditions. SCR technologies utilizing an ammonia-based reductant, such as aqueous urea, have shown potential in achieving high NOx conversion with minimal fuel economy penalty. Moreover, Selective Catalytic Reduction with ammonia as the reductant has been used extensively for stationary source NOx control. The high selectivity of ammonia for reaction with NOx in high O2 environments makes SCR attractive for use on diesel vehicles. Compared to ammonia, aqueous urea is much easier to use onboard a vehicle. SCR catalysts are typically composed of a ceramic flow-through monolith with washcoat or a solid catalyst extrudate. Base metal/zeolites are preferred for vehicle applications due to their superior performance at the exhaust gas temperatures of diesel vehicles and high temperature durability.
Although, current SCR devices and PM filter work reasonably well, separate NOx and PM devices increase system complexity and system cost. Moreover, thermal management of the separate systems is more difficult and negatively impacts fuel economy.
Accordingly, there exists a need for improved SCR and PM devices with reduced system complexity and increased fuel economy.